Gravimeters have an inherent measurement error, usually referred to as a bias, which in general drifts randomly over time. In well-built instruments the bias drifts slowly so that it is possible to complete a measurement, or sometimes several measurements, before the bias has drifted significantly. However, even when it is possible to take several measurements before the bias has drifted too far, comparing measurements taken at different times can require a complex scheme of calibrations.